System controls of coastal and open ocean oxygen depletion

Highlights

• The epoch of the Anthropocene, a period during which human activity has been the dominant influence on climate and the environment, has witnessed a decline in oxygen concentrations and an expansion of oxygen-depleted environments in both coastal and open ocean systems since the middle of the 20th century.

• This review paper provides a synthesis of system-specific drivers of low oxygen in a range of case studies representing marine systems in the open ocean, on continental shelves, in enclosed seas and in the coastal environment.

• Identification of similar and contrasting responses within and across system types and corresponding oxygen regimes is shown to be informative both in understanding and isolating key controlling processes and provides a sound basis for predicting change under anticipated future conditions.

• Case studies were selected to achieve a balance in system diversity and global coverage.

• Each case study describes system attributes, including the present-day oxygen environment and known trends in oxygen concentrations over time.

• Central to each case study is the identification of the physical and biogeochemical processes that determine oxygen concentrations through the tradeoff between ventilation and respiration.

• Spatial distributions of oxygen and time series of oxygen data provide the opportunity to identify trends in oxygen availability and have allowed various drivers of low oxygen to be distinguished through correlative and causative relationships.

• Deoxygenation results from a complex interplay of hydrographic and biogeochemical processes and the superposition of these processes, some additive and others subtractive, makes attribution to any particular driver challenging.

• System-specific models are therefore required to achieve a quantitative understanding of these processes and of the feedbacks between processes at varying scales.

Abstract

The epoch of the Anthropocene, a period during which human activity has been the dominant influence on climate and the environment, has witnessed a decline in oxygen concentrations and an expansion of oxygen-depleted environments in both coastal and open ocean systems since the middle of the 20th century. This paper provides a review of system-specific drivers of low oxygen in a range of case studies representing marine systems in the open ocean, on continental shelves, in enclosed seas and in the coastal environment. Identification of similar and contrasting responses within and across system types and corresponding oxygen regimes is shown to be informative both in understanding and isolating key controlling processes and provides a sound basis for predicting change under anticipated future conditions. Case studies were selected to achieve a balance in system diversity and global coverage. Each case study describes system attributes, including the present-day oxygen environment and known trends in oxygen concentrations over time. Central to each case study is the identification of the physical and biogeochemical processes that determine oxygen concentrations through the tradeoff between ventilation and respiration. Spatial distributions of oxygen and time series of oxygen data provide the opportunity to identify trends in oxygen availability and have allowed various drivers of low oxygen to be distinguished through correlative and causative relationships. Deoxygenation results from a complex interplay of hydrographic and biogeochemical processes and the superposition of these processes, some additive and others subtractive, makes attribution to any particular driver challenging. System-specific models are therefore required to achieve a quantitative understanding of these processes and of the feedbacks between processes at varying scales.

Introduction

The global ocean is increasingly modified by human activities and one of the important responses to these changes is the deoxygenation of coastal and open ocean waters (Breitburg et al., 2018). Low oxygen environments in the ocean have existed through geological time in coastal and ocean systems where oxygen sinks exceed sources of oxygen at varying time scales (Fennel and Testa, 2019). More specifically, the proposed epoch of the Anthropocene, a period during which human activity has been the dominant influence on climate and the environment, has witnessed a decline in oxygen concentrations and an expansion of oxygen-depleted environments since the middle of the 20th century (Breitburg et al., 2018). This loss of oxygen from the ocean serves as a major stressor, affecting biogeochemical cycles and marine ecosystems, and may also interact with other stressors leading to synergistic impacts (Gruber et al., 2011; Bopp et al., 2013).

The recent collation of global open ocean oxygen data shows an average decrease in oceanic oxygen content of more than 2% since 1960, with estimates varying broadly between ocean basins, regions and water depths (Schmidtko et al., 2017, Levin, 2018, IPCC, 2019). Ocean models predict a continued decline in oxygen inventory of 1–7% by the year 2100 (Keeling et al., 2010; Long et al., 2016). Calculations further indicate that rates of deoxygenation are greater in coastal environments than in the open ocean (Gilbert et al., 2010) and in the coastal environment, the number of sites of hypoxia reported in 2008 was considered to have increased by an order of magnitude since the 1960s (Dias and Rosenberg, 2008). These trends of deoxygenation caused the Intergovernmental Oceanographic Commission (IOC) to establish the expert group, the Global Ocean Oxygen Network (GO₂NE) in 2016, tasked with improving our understanding of deoxygenation and its impacts and providing a global and multidisciplinary appraisal of the problem. The undertaking of this review paper was prioritized by the GO₂NE in an effort to synthesize our knowledge and to provide an inclusive perspective of the drivers of deoxygenation in both coastal and open ocean systems. Understanding how and why oxygen varies in space and time is of fundamental importance as this variability shapes the biogeochemical and ecological structure of marine systems.

System sources and sinks of oxygen are dictated by a number of physical and biogeochemical processes, and their imbalance may drive deoxygenation (Peña et al., 2010; Testa and Kemp, 2011; Friedrich et al., 2014; Fennel and Testa, 2019; Oschlies et al., 2018). In surface waters, oxygen concentrations are strongly dependent on air-sea exchange, whereas in bottom waters, particularly in shallow waters, exchange across the sediment–water interface is often significant. Advection by currents is responsible for the flux, both lateral and vertical, of oxygen across system boundaries, and mixing and diffusion are also important physical processes particularly in determining the vertical distribution of oxygen. Therefore, long water residence time and intense stratification tend to isolate water masses from oxygen supply. Biogeochemical processes that affect oxygen concentrations include the production of oxygen and organic matter through photosynthesis and their consumption through respiration and remineralization. Consequently, surface waters usually contain high concentrations of oxygen as a result of photosynthesis and atmospheric exchange. However, in subsurface waters, photosynthesis is diminished and oxygen sinks may begin to dominate supply. Oxygen consumption typically occurs through the oxidation of organic matter in both the water column and sediments, largely by means of aerobic respiration, but as oxygen concentrations are depleted, a sequence of microbially-mediated anaerobic respiratory processes may dominate using electron acceptors other than oxygen. Nevertheless, regardless of aerobic or anaerobic pathways, respiration and the ultimate consumption of oxygen is determined by the supply of organic matter.

Although the independent drivers of oxygen production and consumption in the coastal and open ocean environments are generally known, deoxygenation results from a complex interplay of these hydrographic and biogeochemical processes and there is a clear need for a better mechanistic understanding of oxygen depletion on a variety of scales and across systems. The superposition of these processes, some additive and other subtractive in the net outcome of hypoxia or anoxia, makes attribution to any particular driver challenging. Model development is therefore a likely requirement for providing a quantitative assessment of the contribution of these different processes to observed changes in oxygen, which will in turn also facilitate improved prediction. This paper seeks to provide a review of system-specific drivers of low oxygen in a range of case studies representing marine systems in the open ocean, on continental shelves, in enclosed seas and in the coastal environment (Fig. 1.1). Identification of similar and contrasting responses within and across system types and corresponding oxygen regimes is shown to be informative both in understanding and isolating key controlling processes and, ultimately, in predicting changes in oxygen under past and anticipated future conditions.

Case studies were selected to achieve a balance in system diversity and geographic coverage (Fig. 1.1). Each case study sets out to describe system attributes, including the present-day oxygen environment and known trends in oxygen concentrations over time. Central to each case study is the identification of the physical and biogeochemical processes that serve to transition each system from net autotrophy to net heterotrophy, thereby leading ultimately to either hypoxia or anoxia. Finally, comment is provided with each case study on the susceptibility of each system to further deoxygenation particularly where predictions are available through model simulations and output. The value of within and across system comparison in guiding future observation and modelling needs is also demonstrated.

While the need to use a single unit for oxygen concentration remains a priority, the case studies below express oxygen concentrations in a variety of units as determined by their use in the original publication. For ease of conversion: 1.0 ml l⁻¹ = 1.4 mg l⁻¹ = 44.66$\mu$ mol l⁻¹ O₂; and 1 µmol l⁻¹ = 0.9737 µmol kg⁻¹ at seawater density of 1.027 g cm⁻³. In some cases, the partial pressure of oxygen (pO₂) is presented and expressed in kilopascals: at 100% saturation, pO₂ = 21 kPa = 21 µM at 10 °C.